Gamma Ray Bursters:
Unexplained Lights in the Sky

Gamma
ray
bursters were first noticed in the mid-1960s by military scientists
looking at
data coming in from the new Vela satellites that had been lofted to
help verify
whether the Russians really were abiding by the Test Ban Treaty of
1963. Since
nuclear explosions are highly energetic events, they release gamma
rays. The
new space technology could scan Earth for bursts of gamma rays, and
thus
discover when tests were carried out, and how big the blasts were. The
Cold War
was in full swing, both sides were arming for Mutually Assured
Destruction, and
it was essential to know just how far the other side had gotten.

It's easy to imagine the
consternation
at headquarters when word came in that not only must the Russians be
cheating,
they already had nukes in outer space! But the idea that the Russians
could
have developed space-based nuclear capability without any word leaking
back to
the U. S. was so implausible that the data were checked and rechecked.

By
1967 the military knew that
these
strange bursts came from beyond the solar system. How far beyond, or
why, is
still unknown. The bursters were declassified in 1973, and their
distribution
in the sky was and continues to be mapped (Fig. 1), and their energies
recorded, but that is all we know about them. Speculation abounds, of
course,
and includes violent events associated with neutron stars, black holes,
or even
antimatter.

However,
all these ideas
require
tailored assumptions, and one idea that may fit hasn't even been
considered.
The first notion, in the 1960s, that these were someone's spaceships,
may turn
out to be right -- except that they don't belong to the Russians. Given
the
characteristics of bursters, the hypothesis of alien spaceships is as
plausible, perhaps more so, than some of the other ideas put forward.
Of
course, that requires the acceptance of aliens as plausible, and
therein, no
doubt, lies the problem.

The assumption that aliens
are
improbable doesn't really appear to square with current knowledge.
Astronomical
data indicate increasingly that planets are common around sun-like
stars. Water
is a reasonably common molecule, and the likelihood of liquid water at
the
appropriate distance from a star is good. Paleontological data show
that life
arose almost the (geological) minute the last heavy meteor bombardment
stopped.
Indications of life appear so quickly that some think it may evolve
very easily
in liquid water, and may even have done so several times on Earth,
dying out
during successive heavy bombardments until they finally stopped. Once
life
arises, evolution to fill the various available niches appears
inevitable. If
none of these events are unlikely, the only major uncertainty is
whether
tool-use is likely to advance to the level of interstellar technology.
In my
view the probabilities imply that those who will not consider the
possibility
of alien technology are the irrational ones, not vice versa.

I'll go into the pros and
cons of the
"acceptable" ideas, and then turn to the "unacceptable"
idea: that, just maybe, some of the bursts come from the interstellar
engines
of a starfaring people.

Gamma Background

Gamma
rays are the most
energetic form
of light, with shorter wavelengths than ultraviolet and x-rays, and
with
energies between 100 kiloelectron Volts (keV), or 105 eV, up to 1015
eV. A
large dose of light with 1015 electron volts of energy would not merely
tan,
but would kill a gamma-sunbather in moments. These highly energetic
photons are
produced by interactions between fields (magnetic, gravitational,
electric) and
high energy charged particles (electrons, protons, or charged subatomic
particles). The fields energize and constrain the particles, forcing
them to
interact, resulting in high energy collisions that release specific
subatomic
particles and photons, including gamma rays, with characteristic
energies. The
particles and photons released allow scientists to work backward and
deduce
precisely the reaction that produced that particular pattern. For
instance, if
a proton and proton collide (also referred to as positive hydrogen
ions) they
can start the fusion reaction sequence diagrammed in Figure 2.

Figure
2. Left:
proton-proton chain
reaction resulting in protons, helium and energy. Two protons with one
neutron
each fuse to form a proton with two neutrons (deuterium ion), a beta
particle,
a neutrino, and 0.42 MeV of energy; an added proton can then fuse with
the
deuterium ion to produce a helium nucleus with three neutrons, a
photon, and
5.49 MeV of energy; and in the final step the two helium nuclei fuse to
form an
ordinary helium nucleus with four neutrons (alpha particle), two
protons, and
12.8 MeV of energy. This is the process that generates light and heat
in stars
like the sun. Right:
Tracks made in a cyclotron by electron -
positron
pair production from gamma ray bombardment. In this case the gamma ray
excited
an existing electron. The reaction also works backward to produce gamma
rays
when electrons and positrons annihilate each other.

The
megaelectron volts of
energy
produced could energize a photon to any point on the spectrum,
including gamma
ray, or the energy could be dissipated in other ways, as heat for
instance. As
another example, if a positron, which is an antimatter electron, and an
electron collide, they annihilate each other completely and release
nothing but
energy in the form of photons at 511 keV. The reaction on the extreme
right in
Figure 2 is the inverse, where a gamma ray's energy generates a
positron.

Gamma
rays are produced in
nuclear
reactions, including fission in nuclear power plants or radioactive
decay,
fusion in nuclear bombs or stellar interiors, reactions in cyclotrons
where the
particles are speeded and constrained by magnetic fields, and more
exotic
events such as matter- antimatter annihilation.

Our atmosphere is not fully
transparent
to any wavelength shorter than visible light, whether ultraviolet,
x-ray, or
gamma, and hence astronomical objects emitting gamma rays could only be
studied
easily once orbiting satellites were available. The earliest
ground-based
studies were carried out using Cherenkov radiation, secondary radiation
generated in the atmosphere by collisions between molecules and
incoming gamma
radiation, but both the number and quality of observations were
necessarily
limited. The state of knowledge advanced with high altitude balloon
studies,
with incidental data from spacecraft launched for various purposes,
such as
solar observation, and with sporadic data from satellites launched to
studyhigh
energy phenomena, but only for a few days at a time. The real explosion
of
information came with orbiting satellites able to perform continuous
observations. There are currently two civilian satellites monitoring
these
frequencies: the Russian Granat, launched in 1989, and the US Compton
Gamma Ray
Observatory launched in 1991.

Burster Background

The
Compton GRO has several
instruments
on board for studying the high energy universe, but the appropriately
named
Burst and Transient Source Experiment (BATSE) in particular finds gamma
ray
bursters. Its eight modules facing out in eight directions conduct a
continual
all-sky survey (except where its view is blocked by Earth), recording a
burst
candidate whenever two or more detectors simultaneously note the gamma
count
rising more than 5.5 standard deviations above the background. As a
practical
approximation, this means there is much less than a one percent chance
that
BATSE is mistaking background gamma rays for a burst event, and that
the
background level at the time of an event must be taken into account
when
determining its intensity. This instrument does not make images; it
records the
energy, duration, and direction of burst events, and allows spectra to
be
calculated.

BATSE
has expanded our
knowledge of
gamma ray bursters enough to make us realize how little we know about
them.
About 800 bursts occur per year, and the typical burster blooms in a
few
seconds, radiating nothing detectable except gamma rays, and disappears
after a
few seconds or minutes. For a high energy event to happen so quickly,
the
source has to be very compact, theoretically no more than a few hundred
kilometers across, or it would take too long for the reaction to
propagate
through the whole mass.

In
the two years of data so
far
available, BATSE has shown us that bursters have the following
characteristics:

They are isotropic, that
is they are distributed throughout the
sky.

There are fewer weak
sources than expected.

They have differing time
profiles.

They do not repeat, that
is, bursts are always coming from
different
places, never from the same place repeatedly.

There is no other
associated radiation, with a few exceptions.

Theories about Bursters

Isotropic Distribution

The distribution of bursters
throughout
the sky (Fig. 1) is really what has astrophysicists scratching their
heads.
They know intense nuclear reactions produce gamma rays, they know that
the
energies and field strengths needed are most commonly found on neutron
stars,
and so they were pretty sure that the bursts should be associated with
neutron
stars in some way.

Some
scientists favored the
idea of
binary systems, with a neutron star pulling in ionized hydrogen and
helium from
a lower-mass companion along its magnetic fields, funneling the gas to
its
poles where the gas builds up until explosive helium fusion finally
starts,
releasing a sudden burst of gamma rays. Others suggested that random
chunks,
such as asteroids or passing planets, attracted by the star's immense
gravity,
accelerate toward it so rapidly that the final collison happens at a
significant fraction of the speed of light, generating the rays. Some
even
suggested that the chunks could be antimatter. Yet others thought it
might be
the sudden release of energy from a massive neutron starquake.

The
problem is that neutron
stars,
strange as they are, occur within galaxies, like their more ordinary
siblings.
Since we are near an edge of our galaxy, and the galaxy is flattened,
events
confined to the galaxy are concentrated in the strip formed by the
Milky Way.
The early data on gamma ray bursters showed them more or less evenly
distributed in the sky, but it was assumed this was only because the
data were
too patchy to show the true, disk-like pattern. Once BATSE showed that
bursters
really were distributed throughout the sky, all the neutron star
theories no
longer fit the data.

There
are two ways to have an
even
all-sky distribution. Events that are so close to us they fit within
the one
thousand light year thickness of the galaxy in our neighborhood will
appear to
be omnidirectional. On the other hand, events on a cosmological scale
and out
at the edge of the universe will also appear to be evenly distributed.
Of
course, if we receive a given amount of light in a gamma ray burst from
right
next door, the source must be comparatively weaker than if that same
amount of
light was part of a burst whose light had been spread over the whole
universe.
So, to understand what sort of events are causing the bursters, it is
essential
to know how far away they are.

Distances to Bursters

No direct evidence exists at
this point
about how far away gamma ray bursts really are. Distances have been
estimated
using distribution data, which suggest that bursters occur either
within about
300 light years of Earth, or within 150,000 - 300,000 light years (out
at the
Magellanic Clouds and beyond to the halo of dark matter assumed to
control the
gravitational dynamics of our galaxy), or out at billions of light
years. The
first number is well within the local thickness of the galactic disk
(about
1000 light years). The second range of numbers refers to a distance
well beyond
the visible stars of the galaxy, whose globular clusters extend out to
40,000
light years, but within the range both of the Magellanic clouds
(150,000 light
years) and the halo of dark matter that is assumed to control the
gravitational
dynamics of the galaxy. These estimates suggest where the bursters
aren't, but
for a better idea of where they are, we will have to wait till spectral
data
are available.

Redshifts provide a direct
indication
of distance, but their calculation requires accurate spectra. BATSE
records
energy levels, which are raw data whose distribution, duration and
strength can
be interpreted directly and hence have been studied right from the
first
availability of the data. BATSE also records spectra, that is, the full
complement of wavelengths making up the light, but transforming that
data into
something interpretable requires calculation. Theoretical background
for some
of the calculations is still being worked out, and the programming to
calculate
gamma ray spectra has only recently been developed, so that very few
are
available for study, and those that are available come from bright
bursters. It
is possible that statistical studies of many more observations gathered
over
the next few years may allow an overall red-shift to be estimated.

What
few spectral data have
been
acquired so far speak against the idea that light from the bursters has
travelled very far. No evidence of red- shifts has been found up till
now,
which is completely implausible for light travelling billions of years.
Cyclotron resonance, a somewhat delicate pattern caused by magnetic
fields at
the gamma ray source, is unlikely to be visible in energy that has
travelled
across the whole universe, and the positron annihilation peak would no
longer
appear at 511 keV (due to redshift). Yet both Russian and Japanese
satellites,
as well as balloon studies, have detected these two patterns. BATSE
data have
not yet confirmed them, but spectral analysis has only just started,
and it is
too early to say that these patterns won't be found. What has been
found in the
BATSE data are spectral patterns that break at relatively low energies
(Fig.
3), which is said to occur in high energy reactions. A reaction with
sufficient
energy close to home could be something relatively small, like a
antimatter
explosion limited to a few tons of mass, but an event at the edge of
the
universe would have to be so energetic as to be physically impossible.
The low
energy breaks in BATSE spectra suggest that at least some of the
sources are
within about 3000 light years of Earth.

Gamma
ray burster data require
a compact
source, and they suggest a close source, whose emissions are limited to
the
highest energies of light, and which has some kind of magnetic
containment. But
instead of working on testing the obvious hypothesis of alien engines
(perhaps
because the probability of getting funding is considerably lower than
the
likelihood that aliens exist), scientists have concentrated on natural
explanations.

Neutron star theories

Astronomers
have not really
contemplated
the close-to-home hypothesis, because no known astronomical phenomenon
in our
neighborhood could generate gamma ray bursts without other associated
radiation. Instead many astronomers have concentrated on modifying
theories
involving neutron stars so that they fit the observed distribution.

The
true diehards have
modified the
local neutron star hypothesis to fit an isotropic distribution. Instead
of
neutron stars within the galaxy, they postulate high-speed neutron
stars moving
fast enough to escape galactic gravitation. By the time the stars reach
150,000
or more light years from the main galactic disk, which is still
considered
"local" in cosmological terms, they are assumed to have aged enough
to change from pulsars to gamma ray bursters.

The
local neutron star theory
requires
many assumptions tailored to fit the theory. Pulsars have not been
observed as
strong gamma ray sources, and adherents to this theory assume that
exceptionally strong magnetic fields on some stars turn them into
bursters. The
theory requires a very unusual initial population of stars to form the
unusual
high velocity pulsars, a population unlike any currently seen, and
whose
possible historical existence is not supported by the distribution of
elements
in the galaxy. Also, since our galaxy is not unique, similar halos of
pulsars/bursters should be seen around other galaxies, but they are not.

A
galactic halo of neutron
stars has
enough evidence against it that many astrophysicists lean toward a
cosmological
hypothesis, where the bursters come from billions of light years away.
Mere
infalling matter cannot generate enough energy to create a gamma ray
burst
detectable from that far, so the hypothesis shifts to collisions of
whole
neutron stars, or of neutron stars and black holes, or of "strange"
stars composed of quarks. These can indeed generate the energy required
to make
a burst visible to us, (though not enough to explain the energy breaks
discussed above), and it is possible that there would be enough such
events,
given the vastness of the universe, to account for the observed
frequency of
bursters.

There
are, however, problems
with the
cosmological collisions idea. The first is the same problem of
location,
transposed to a different key. If neutron stars, or black holes, or the
like
are involved, the bursters ought to occur more commonly in the
neighborhood of
clusters or superclusters of galaxies. They don't appear to, though
additional
data may indicate some clustering that is not yet evident.

Lack of associated radiation

Another objection to the
collision
hypothesis is the lack of any other radiation associated with the gamma
ray
bursters. Colliding stars or black holes would have to release one
percent,
possibly more, of their nuclear energy as pure gamma rays. This is
unlikely in
any case. Nor have theorists arrived at a consensus concerning the
observable
characteristics of such a cataclysm, that is, no-one knows what it
would look
like if it happened. And, in addition, where is the other 99% of the
energy? We
should also be seeing UV bursters, and even visible light bursters,
sometimes
all coming from the same event. But we don't.

One explanation advanced for
the lack
of other radiation is that the extremely violent collisions supposed to
generate bursts release a blast wave travelling out at a large fraction
of the
speed of light. When these relativistic particles hit the interstellar
medium,
kinetic energy is transformed to a tremendous increase in radiative
energy,
such that the photons radiated by the excited material are strongly
increased
in energy, or blue-shifted. Light of all wavelengths is blue-shifted
until
nothing but gamma rays is present.

The
astrophysical theoretician
Paczynski
has pointed out that relatively little radiation, including gamma
radiation,
might escape from neutron star or black hole collisions, since their
gravity is
so strong and their substance so dense. To overcome this problem,
Paczynski
suggests that bursters occur from collisions between "strange" quark
stars, which would theoretically be expected to release gamma rays.

A
few times bursters have
lasted long
enough to allow other instruments to be focused on them. On one
occasion a
multi-day burster led to the identification of a faint source at
optical and UV
wavelengths, showing what appears to be a gamma-ray nova. Perhaps,
associated
radiation would be found in other bursters if we could just get a long
enough
look at them. But the very oddity of this particular source suggests
that the
nature of the others would be quite different.

Weak source deficit

Yet
another difficulty is that
gamma
rays interact with infrared photons in the intergalactic medium, losing
their
energy. We ought, logically, to be seeing a large number of very weak
sources,
if they're coming all the way from the edge of the universe, because so
many
would have been attenuated on the way. But in fact, the opposite
pattern
occurs. There is a deficit of weak sources, not an excess (Fig. 4).

Fig. 4. Number versus
strength of gamma ray bursts. Strong
bursters toward
100 on the x-axis, weak toward 1. Dashed line shows expected numbers of
bursts
for any given strength in a perfectly spherical distribution. The
number of
weak sources is somewhat too low for such a distribution, and the
number of
strong sources somewhat too high (though the sample size of the latter
is
small, making conclusions about strong sources suspect). (From Meegan
et al.)

The
weak source deficit has
another
implication as well. It means we must not be at the center of burster
distribution. To visualize this, imagine yourself as the guest of honor
at the
world's most enormous fireworks display. All around you, as far as the
eye can
see, fireworks are being set off. Right out to the horizon, you can see
progressively fainter and fainter bursts. If, on the other hand, you
approached
one inner edge of this massive ring of fireworks, you'd have more very
bright
sources near you, and you wouldn't be able to see the weakest sources
furthest
away across the circle. This distribution is what BATSE has seen so
far: a
slight excess of strong sources, and a deficit of weak sources. If
we're not at
the center of the bursts, then they can't be happening out at the edge
of the
universe, because we are at the apparent center of everything happening
that
far away. An alternate explanation could be that different types of
sources
generate gamma bursts, and that some types are not evenly distributed.

Time Profiles

Bursts
share the
characteristic of
brevity, as their name implies, usually lasting from one to a few
hundred
seconds. Everything else about their timing varies (Fig. 5), which
supports the
idea of different causes generating the bursts. Variables include: the
initial
phase when they build up intensity, known as rise time, the peak
energy, the
number of peaks, their duration, and the duration of the whole burst.

The
duration of rise time,
typically as
long as a few seconds, is yet another problem for the colliding neutron
star
hypothesis. The energy release from such a collision would be over in a
few
milliseconds and would have a much faster rise time. The energy
structure of
the whole burst, with various levels of energy arriving at various
times, is
also too broad for what is expected in such a collision. Paczynski
suggests
that possibly the collision results in the brief existence of an
infalling
accretion disk that accounts for the longer lifetime of gamma bursts,
though
still not for their "long" rise times.

Other Possibilities

So
far we know that gamma ray
bursts are
found throughout the sky, must come from very compact objects, and
don't appear
to have any other associated radiation. We also know of no natural
process that
might have this set of characteristics. Prevailing theories that
attempt to
press the bursters into a natural mold seem to require as many
favorable
assumptions as a government budget.

What
else could we consider?
Could we be
seeing hapless chunks of matter, or antimatter, bumping into cosmic
strings?
Because gamma rays are generated in strong fields, the potentially
superconducting ends of the strings would have to be involved,
according to
Paczynski. However, cosmic strings pack so much energy in such a small
space
that all the energy would be released in a millisecond, according to
theory,
and the burster time profiles do not fit well. Still, further analysis
of
bursters from the perspective of string theory may bring interesting
things to
light.

A
local phenomenon that forms
a
spherical shell around us is the Oort cloud of comets, but the only
source both
small enough and energetic enough to generate the observed bursters at
that
distance would be antimatter annihilation. Astrophysicists are very
reluctant
to imagine free-flying chunks of antimatter so close to home,
especially since
none has ever been seen in the inner solar system.

However,
an even stranger idea
may fit
the facts at least as well as the natural explanations. At least some
of the
gamma ray bursters are local, and are due to antimatter annihilation,
but are
not free-flying. This antimatter is carefully contained in magnetic
fields
aboard starships accelerating for interstellar journeys. The ships may
be
accelerating to superlight speeds, or they may be the slow-moving
barges of the
fleet, ferrying tools or medicines or foods at sublight speeds to young
colonies.

Just
as cargo planes and
barges use
different fuel, not all the spaceships may burn exactly the same fuel
in
exactly the same way, and the rate at which they burn may depend on the
cargo,
accounting for different time profiles. They might use positrons and
electrons,
protons and antiprotons, hydrogen and anti-hydrogen, or maybe something
even
more exotic. Since these are not accidental chunks of antimatter, it is
not
surprising that similar chunks haven't appeared in our solar system.
The number
of bursts accounted for by this "strange" hypothesis could be
anywhere from few to many. Who knows, maybe this alien civilization has
just
started interstellar colonization and can only afford a few flights a
year.
Whether it is many or few, alien spaceships are potentially distributed
widely
in our local space, would account for the lack of other associated
radiation,
and would provide a plausible source of compact, high energy events not
seen
within our solar system (Table 1).

It
is worth noting that
theorists
suggest that bursters from the edge of space may be subject to
gravitational
lensing, and that the arrival of identical bursts separated in time
would be
proof of a distant origin. However, an equally good reason for
identical bursts
from the same place is that a ship has departed from the same port.
Thus events
that actually support the alien hypothesis may be subsumed under the
cosmological one. A similar problem could arise in the calculation of
overall
red-shift, mentioned earlier. If alien ships account for only a few
bursters,
they could be lost in the data. Pooling data from different events
should only
be done when they are known to come from the same source, though then,
of
course, if their source is known, the need to pool the data disappears.

The
major problem with this
idea of
alien gamma ray ships, is that one would expect the aliens to have
ports, and
therefore one would expect bursters to be concentrated there. The
isotropic
distribution of the bursters is a problem yet again. One would also
expect
similarity in some of the time profiles, though if alien ships account
for only
a few bursters, it may take time for similarities to become evident. In
this
connection it would be helpful if time profiles, and spectra when we
get them,
were mapped on to the distribution, so that possible local areas of
similarity
were evident.

Another
possible problem is
that
repeated bursts have never been observed. The aliens may require only
one
massive burst for their purposes, but logically it would seem that a
series of
bursts would allow for more controlled acceleration and a smoother
ride. But
who knows, they may be speed demons with the ability to control
inertial
effects; or jumps to hyperspace may not work like that....

One
question, initially raised
by Fermi
when the presence of alien civilizations was being seriously
considered, is
"Why haven't they visited?" I can think of several reasons. One is
that space is too vast for the probability of bumping into Earth to be
good.
However, if the bursters all around us are ships within 300 light
years, that
is not a good answer. Another possibility is that Earth is a backwater.
After
all, Okiwi Bay in New Zealand may be an extraordinarily beautiful
place, and
not all that far away, but very few people have ever visited it, or
even have a
map detailed enough to find it. And finally, another answer may be,
"They
have visited, we just don't admit it." Though almost all UFO sightings
have natural explanations, there are a few that cannot be explained
away.

The
wonderful thing about
mysteries is
the possibility they give us of seeing new wonders. Gamma ray bursters
are sure
to show us worlds undreamed of, -- and some of them may even be
populated.